[X86] Remove what appear to be unnecessary uses of DCI.CombineTo

[X86] Remove what appear to be unnecessary uses of DCI.CombineTo

CombineTo is most useful when you need to replace multiple results, avoid the worklist management, or you need to something else after the combine, etc. Otherwise you should be able to just return the new node and let DAGCombiner go through its usual worklist code.

All of the places changed in this patch look to be standard cases where we should be able to use the more stand behavior of just returning the new node.

Differential Revision: https://reviews.llvm.org/D49569

llvm-svn: 337589